Bearing finding equipment, such as time difference of arrival (TDOA) systems and equipment for laboratory measurements of frequency and time delay in high speed circuits require high resolution time interval measurement. D. Martin (Electronic Design, Nov. 24, 1974, pp. 162-167) has discussed the principles of time interval measurements by various techniques.
Two techniques are presently useful for achieving subnanosecond resolution for single event measurements. The first method is a vernier technique which is used in vernier time interval meters. An example of such a meter is the Model 796 time interval meter made by the Eldorado Instrument Co. of Pleasant Hill, Calif. in which a relatively low frequency clock, such as a stable triggerable 100 MHz clock, is used. Because such relatively low frequency logic circuits are used, the measurement time is at least one microsecond and a rate at which new measurements which can be taken is therefore limited to less than one MHz.
R. G. Baron (Proceedings, IRE, January 1957, pp. 21-30) describes the techniques of vernier time measurement by which it is possible to measure non-periodic and asynchronous time intervals accurately. The Baron apparatus utilizes three clock pulse generators (one free running and two startable upon command), three counters to count the pulses produced by respective pulse generators, and two coincidence circuits to define coincidence between pulses of the free running pulse generator and each of the other generators. The Baron interval measurement apparatus, thus, utilizes a relatively large number of components and a complex circuit.
In accordance with the second technique, time spaced pulses of unknown separation and a signal from an oscillator of known period are processed by an AND gate producing a series of pulses. The pulses are then counted by means of a series of frequency dividers. The resolution of this system is .+-.1 period of the oscillator. Since the best dividers presently available (IEEE Spectrum, March 1977, pp. 41-47) operate at a maximum frequency of 4 GHz, the best precision possible by this technique is .+-.0.25 nanoseconds, that is, a resolution of 0.5 nanoseconds.